Lots of the rules for writing non-fiction are just the same as they are for writing fiction ("Put one word after another", 20 February). Whether it's biography, history, astronomy or anything else that comes in the category of factual, you need to be at your desk just as early (or at least as long) as if you live by your imagination. And you need to turn up there every day (unless you are somewhere else, of which more later).

You need to avoid exclamation marks and clichés (like the plague), you should use adverbs as if they were rationed and remember that in real life – which is where non-fiction writers are – people say things, or possibly argue them, or occasionally insist on them. They do not proclaim, or aver, or laugh them (as celebrity magazines would have us believe) – or mull them, for heaven's sake, as I read recently. And if you ever start a sentence with "meanwhile", you have literally lost the plot.

Non-fiction writers have many advantages over fiction writers: the most profound being, as Richard Holmes said when he was writing his biography of the poet Shelley, "At least I always have the man." As I wrote The Thirties: An Intimate History, I felt grateful every day for the fact that I always had the decade, and even on bad writing days there was something I could spend time finding out that I was pretty sure I would be able to use. The crumpled ball of paper syndrome (or its electronic equivalent) still happens, of course, but it can usually be smoothed out and used somewhere.

The other plus point is research – and that brings us to where the being somewhere else comes in, because writing non-fiction books requires months, if not years, in libraries and archives and record offices, reading newspapers, invariably on that mind-numbing interim technology, microfilm. Or talking to experts, or what we now call "witnesses to events", following up leads, interviewing people, listening to tapes, poring over illegible scrawl in letters and diaries that, when you have finally managed to decipher it, might well say nothing more arresting than, "Rained again. Vicar called. Mother had headache."

But at least non-fiction writers are not usually advised to "wear their research lightly", though all should try to, as should rather more novelists. On the whole, what you have found out, you can tell straight on the page, without having to invent some intermediary to be the messenger.

Another advantage is that, if the worst comes to the worst, you can comfort yourself with the thought that, even if a reader finds your book boring, or thinks it is badly written, at least they are bound to have learned something from it. But they might have garnered little from a mediocre novel and be none the wiser about anything.

So, if those are some advantages of being a non-fiction writer, what are the disadvantages? One is the osteopath's bills. Most writers sit cramped hour after hour over their laptops, but when non-fiction writers get up from their desks, it's often to go off to libraries and load themselves up with so many books that they develop aches in muscles they didn't know they had. Another is the photocopying costs. One has to be iron-willed not to photocopy that whole chapter, this entire article, just in case, rather than choosing the bits you really need to transcribe. There are digital cameras, of course, but the British Library won't allow them, and you have to be adept to ensure that you can read more than just the centre third of a page when you upload the image back at home.

Then there is the amount of money it is all too easy to spend at AbeBooks, or increasing the number of books that can be borrowed from the London Library from the statutory 10 to the maximum 40, to assuage that insecure feeling that a long out-of-print biography, or a foxed and forgotten memoir, will provide just the illuminating quotation that will vivify a whole chapter for you.

It is the sources that shackle a non-fiction writer: if something didn't happen, you can't say that it did. You can't write what you would have liked someone to have said, or even improve on how they said it. You can speculate a bit – Hans Magnus Enzensberger does so wonderfully in The Silences of Hammerstein, but then he is also a poet. Most readers get irritated if a writer presumes an omnipotence and tells them what someone thought when they can't possibly know. And the shackles are material, too: Lady Antonia Fraser invented her lady detective, Jemima Shore partly so that she could sit in the sun with Harold Pinter and still be writing, without needing to have shelves of books and tottering piles of paper surrounding her, as she did when producing her historical biographies.

Literary festivals and bookshop events are harder work for the non-fiction writer: the audience expects a proper talk, an argument – maybe even a PowerPoint presentation. You can't get away with just reading a few pages of lovely prose from your latest book, as a fiction writer can. But perhaps the real killer is that while fiction is read and reread over generations – centuries even – non-fiction, even really fine non-fiction, with a very few exceptions, becomes all too soon outdated and overtaken. Some writers cross the boundary, of course: novelists do sometimes turn their hands to non-fiction, and vice versa. But not often, and not usually very successfully. I don't think I could. And I am not sure that I want to. My curiosity is boundless, and I don't want to make up characters and imagine situations; I want to find out about what exists and try to write about that as fluidly and as compellingly as if it were a work of fiction.